1. Field of the Invention
The invention relates in general to the fabrication of semiconductor integrated circuits (ICs), and more particularly to a method for forming a metal plug with good step coverage.
2. Description of the Related Art
In general, the fabrication of metal plugs comprises steps of forming a trench opening in a dielectric layer and forming a metal material in the trench opening. Since adhesion between the metal and the dielectric layer is not ideal, a titanium/titanium nitride (Ti/TiN) layer used as a glue/barrier layer is often formed between the metal and the dielectric layer to enhance the adhesion. As the IC devices are continuously sized down to a line width of 0.25 .mu.m, the trench opening has a large aspect ratio so that the glue/barrier layer is formed using chemical vapor deposition (CVD) with a better step coverage than other methods.
However, the structure of the glue/barrier layer formed on the bottom of the trench is worse than the glue/barrier layer formed on the sidewall of the trench. The worse structure means that grains of the glue/barrier layer are not uniform or that the glue/barrier layer is amorphous. The structure described above has a high resistance and decreases the adhesion for a metal layer formed in the trench during a following step. A plasma treatment comprising H.sub.2 /N.sub.2 is performed at the surface of the glue/barrier layer after depositing the glue/barrier layer to enhance the adhesion.
FIGS. 1A-1C are schematic, cross-sectional views showing the conventional process of forming a metal plug. As shown in FIG. 1A, a substrate 101 having a first metal layer 104 formed thereon is provided. A dielectric layer 102 is deposited on the substrate 101 and defined to form a trench opening 103 that exposes the first metal layer 104. A titanium layer 105 is deposited conformal to the profile of the dielectric layer 102. A titanium nitride layer 106 is deposited on the titanium layer 105. The titanium layer 105 and the titanium nitride layer 106 are used as a glue/barrier layer. A H.sub.2 /N.sub.2 plasma treatment is performed to make the titanium nitride layer 106 more tenacious.
The titanium nitride layer absorbs moisture easily. During device fabrication, a step may be performed after waiting a period of time after finishing the foregoing step according to actual conditions. For example, there may not be enough machines to perform the foregoing step on devices at the same time. If a metal plug is filled in the trench a while (about 2-24 hours) after the titanium nitride layer is formed, the titanium nitride layer is exposed to the air and absorbs moisture. After absorbing moisture, the titanium nitride layer becomes incohesive and has a higher resistance than a titanium nitride layer without moisture therein. As shown in FIG. 1B, the titanium nitride layer 106a absorbs moisture 107 from the air. This increases the resistance of the titanium nitride layer 106a and decreases the adhesion of the titanium nitride layer 106a.
In FIG. 1C, a tungsten layer (not shown) is formed on the titanium nitride layer 106a to fill the trench opening 103. The tungsten layer other than what is in the trench opening 103 is removed to form a tungsten plug 109 using the titanium nitride layer 106a as a etching stop layer. The tungsten layer has bad step coverage because of the adhesion of the titanium nitride layer 106a, so that the tungsten is deposited faster at the top portion of the trench 103 than at the bottom portion of the trench 103. A hole 110 forms within the tungsten plug 109 while depositing the tungsten plug 109 in the trench 103. The hole 110 decreases the performance of IC devices comprising the tungsten plug therein.